Low-slope Roofs Research Articles

Factors to Consider in Developing Conceptual Scopes of Repair for Common Low-Slope Roofing Assemblies

Forensic engineers are commonly asked to develop conceptual scopes of repair as part of their work. Many factors impact these recommendations, including building codes, construction feasibility, manufacturer assessment, and installation requirements. In addition, the conditions present on and within the existing roof surfaces can limit the repairability of a commercial roof assembly such that removal and replacement of the entire roof section is the appropriate or only feasible repair option. This paper will focus on common limitations to be considered when developing a conceptual scope of repair for common commercial roof systems, including single-ply membranes, built-up roofing, metal panel roofing, spray polyurethane foam roofing, and the application/maintenance of roof coatings. It will also discuss an assessment methodology that can assist in developing a broader understanding of the condition of the roof surfaces.

Forensic Analysis of Roof Deterioration Due to Condensation

Corrosion of the structural steel roof deck of large warehouses resulted in several invasive forensic investigations to determine the cause and mechanism of the corrosion. The warehouse roof investigation presented in this paper is one of many similar warehouses located near the Gulf of Mexico and along the Southern Atlantic Seaboard experiencing corroded roof decks when constructed using fiberboard roof insulation made from sugarcane fiber called bagasse. Consensus exists among individuals studying this phenomenon that the fiberboard became saturated with water, which dissolved formic and acetic acid from the fiberboard. The steel deck corroded when it came in contact with the acidic solution. There was little consensus regarding the mechanism by which water intruded into the roof assembly. Invasive investigation of the low-slope roofs of buildings with various types of insulation (including bagasse fiberboard) and observation of physical commonalities between the instances led to the following conclusion: Water in the roof assemblies did not result from leaks through a compromised roof membrane but rather from humid air infiltrating the roof assembly and condensing on the underside of the roof membrane.

Impact of Solar Mounting Systems on the Thermal Design of Commercial Roofs

Commercial rooftops provide extensive areas representing the desired platform for installing photovoltaic (PV) systems. The combination of roofing assembly and PV system is called photovoltaic roofing assembly (PVRA). Currently, there is a shortage of information about building codes on PV system and roofing assembly integration. The National Research Council of Canada (NRC) developed an industry consortium project to generate codifiable research data on the wind and thermal performances of photovoltaic roofing assemblies. This paper examines the thermal bridging of the PV mounting attachments and its impact on the overall thermal resistance of the roofing assembly. Seven different types of PV mounting fixed attachments were tested in this study. Thermal bridging evaluation of PV mounts was done on the component level as individual mounts were installed through a section of a roofing assembly. The roofing assemblies were designed for a prescriptive thermal resistance of RSI 5.46 m2 K W−1 (R-31 h ft2 °F BTU−1) as per current standard and requirements for Climate zones 4–6. The thermal bridging experiments were conducted on a 1.2 m by 1.2 m guarded hot box at a mean temperature of 24°C. The measured data indicated a decrease in the effective thermal resistance of the PVRA, ranging from 3.3% to 50.0%, compared to the opaque roofing assembly devoid of any thermal bridging. Furthermore, it was found that among fastener-designed PV mounting attachments, the greater the number of fasteners, the more significant the decrease in effective thermal resistance. From the experimental data, chi factors (χ) were developed to support the calculation of point thermal bridging effects on the thermal performance of low-sloped roofing assemblies. Toward codification, efforts are underway to potentially implement these chi factors in the energy codes that could enhance the thermal design of both retrofit and new roof constructions installed with photovoltaic systems.

Wind induced pressures on a large low-sloped gable roof building with parapet

This paper experimentally investigates how parapets with varying heights can alter the flow pattern around a large aspect-ratio low-sloped gable roof building. We examine the benefits and challenges of existing methods and utilize an open-jet wind test section, to accurately simulate real-world atmospheric boundary layer conditions. A model of a low-rise building was fitted with solid parapets of varying heights and tested in a comparative study to understand their impact on roof pressures. The study compares the standard approach to the experimental results and suggests improvements to wind zones and pressure coefficients. The parapet of fourteen percent of the building height is the most efficient at reducing mean, peak, and area-averaged wind loads compared to other sizes. However, the standard provisions underestimate roof pressures for components and cladding design.

Low-Sloped Rooftop Storm-Water Detention Assembly to Mitigate Urban Flooding

Low-sloped roofs such as commercial and high-rise buildings in a dense urban area provide vast “unused” surfaces that can be used to manage storm-water and mitigate urban flooding. Storm-water Detention Assembly (SDA) known as “blue roof” exerts advanced drainage technologies and operational strategies to store-and-release storm-water during and after heavy rainfall events. SDA can reduce peak flow and decrease storm-water run-off volume. However, the SDA application is limited due to the lack of science-based regulatory requirements. This paper introduces SDA, summarizes the benefits and challenges of this system, reviews the existing code specifications on roofing drain systems, and documents the criteria to design and construct SDA. With this systematic approach, the missing links are identified that shall form code change request for possible inclusion in the National Building Code of Canada (NBCC) and National Plumping Code of Canada (NPC). In addition, a road map is stated to calculate and benchmark the drain requirements on low-sloped roofs for a given design climatic load.

Wind loading on a low-slope gabled roof: Comparison of field measurements, wind tunnel data, and code provisions

The current wind load provisions for low-slope roofs in the NBCC were established in the 70 s with wind tunnel experiments using analog data technology. These coefficients have remained almost unchanged. As the wind pressure measurement tools have significantly improved during the past 50 years, verification of the original pressure coefficients is required. The National Research Council of Canada (NRC), in collaboration with the Special Interest Group on Dynamic Evaluation of Roofing Systems (SIGDERS) consortium, conducted a wind load investigation to verify the suitability of the current wind load provisions. The investigation was carried out using both field measurements and wind tunnel simulations. An ideal full-scale representation of the low-slope roof specified in building code was instrumented and continuous data was collected for four years. A duplication of the building was evaluated by two wind tunnels to investigate the effect of critical wind directions. The paper presents the efforts made to make field measurements comparable to wind tunnel data and code provisions. It was observed that records with mean wind a speed above 22 mph (10 m/s) reduce the data variability and maximize reliability, which are important features for the code verification process. The goal of this research is to combine multiple sources of data to verify the suitability of wind load provisions on low-slope roofs. The research focuses on NBCC-2020 as an application. Evidence is presented that the NBCC-2020 provisions for the Roof Edge Zone is currently underestimated and needs to be increased. The study suggests taking the opportunity of Edge provision enhancement to merge Corner and Edge into a unique “Perimeter” load provision. The suggested code change addresses the current underestimation of the Edge Zone coefficients (wind load upgrade), and provides construction simplification to minimize roofing failure associated with labour-induced errors during cladding system installation (constructability upgrade).

Numerical simulation study of wind-induced snow drifting on low-sloped gable roofs

In this paper, an effective diffusion coefficient derived from previous semi-empirical formula and dynamic grid technology combined with inverse distance interpolation algorithm are applied to simulate wind-induced snow drifting on low-sloped gable roofs. The effective diffusion coefficient enhances the diffusion ability of particles near the wall surface, and the inverse distance interpolation algorithm has an explicit format, which determines the influence of the sample point on the interpolation point according to the interpolation distance, it has a low computational expense and weakens the influence of extreme points. Two measurement parameters, δH and δM, are introduced to illustrate the impact of interpolation parameters (power law p and interpolation radius R) on the single grid adjustment and snow cover. By employing these optimisations, sensitivity analyses for simulation parameters are performed, the effects of the saltation layer height, turbulent Schmidt number, falling velocity, time step size, and wind duration on snow distribution are discussed. Finally, the redistributions of snow cover on gable roofs under various wind directions are reproduced, the snow redistribution characteristics and the relationship between roof slope and snow erosion are investigated. The results can provide a reference for predicting snow loads on gable roofs.

Development of Psi Factors for Thermal Bypass Due to Insulation Gaps in Low-Slope Roofing Assemblies

In commercial roofs, the presence or formation of gaps could be due to improper installation, thermal expansion, and dimensional changes in the insulation boards. The heat loss from these gaps could lead to higher thermal transmittance in the roof assembly. The current research study conducted around 70 experiments to investigate the effect of gap height, gap width and gap offset on the thermal transmittance of the roofing assembly. The measured data showed that in a staggered insulation layout with a joint offset of 610 mm (24 in), formation of 6.4 mm (1/4 in) to 12.7 mm (1/2 in) gaps at the insulation joints could contribute to an average decrease of 2% to 9% in the effective R-value of the roof assembly. As the insulation thermal resistance increases or becomes thicker, the thermal losses in the roof assembly increase. Generalized gap impact curves were developed to provide the relation between gap parameters (i.e., gap widths and height) and the thermal performance of the roof assembly. The experimental data were further analyzed using the psi factor approach of linear thermal bridging generating thermal transmittance data to support the calculation of thermal bypass from gaps in the thermal roof design.

Mean Operating Temperature (MOT) of Commercial Roof Assembly and Its Impact on the Energy Performance

In the thermal design of low sloped roofing assemblies, two parameters are overlooked, one is the surface temperature of the roof assemblies which provides the required temperature gradient for heat flow, and the other is the mean operating temperature of the roof assembly, which has direct implications on the thermal performance of the insulation. An in situ field study was conducted in collaboration with Alberta Roofing Contractors Association (ARCA) on their headquarters building located in Calgary, to generate data on the mean operating temperature of the roof assemblies and to determine whether the thermal design of roofing assemblies using conventional methods is an accurate portrayal of in-service thermal performance. For the present study, two roof assemblies insulated with polyisocyanurate insulation, one with a white reflective roof membrane and the other with the black membrane were selected and instrumented. During the monitoring period, the mean operating temperature (MOT) of the roof assembly whether it is calculated as the average of interior and exterior ambient (MOTAIR,) or the average of surface temperatures (MOTSurface), was found to be below 24 °C (75 °F), which opposes the current roof thermal designs that are being designed using label R-value (thermal resistance) of the insulation reported at the mean temperature of 24 °C (75 °F) rather than temperature-dependent thermal resistance. The comparison of two energy transfer theoretical models, QConvention and QMOT, with the measured data indicated that the conventional approach of roof thermal design underestimates the energy performance of the roof assembly on average by 30%. The use of roof surface temperatures and the corresponding temperature-dependent thermal resistance of the insulation as in QMOT has been demonstrated to improve predictions of the energy performance. In addition the loss in thermal resistance due to blowing agent diffusion in polyisocyanurate was evaluated after two years of in situ installation.

Characterization of wind-induced pressure on membrane roofs based on full-scale wind tunnel testing

Flexible roofing systems (e.g., membrane roofs) are widely used in low-rise commercial and industrial buildings, accounting for over 60% of low-sloped building roofs in North America. Despite their wide usage, the effects of roof flexibility are neither thoroughly studied, nor accounted for in current building design codes and standards. To investigate such roof flexibility effects on wind-induced pressure, full-scale testing was conducted at the NHERI Wall of Wind (WOW) Experimental Facility (EF). The mechanically attached roof system (MARS) and partially adhered roof system (PARS), two of the most commonly used commercial roofing systems with different flexibility characteristics, were considered in this study. “U-shaped” pressure taps were used to measure the wind-induced pressure on the membrane roof, and the pressure coefficients were compared to those measured on a plywood roof. The effects of wind direction and wind speed on the wind-induced pressure coefficients of membrane roofs were studied. The results showed that the membrane deformation due to the membrane roof flexibility resulted in a reduction in the peak pressure coefficients as compared to those of rigid plywood roof. This reduction in wind loads is more significant for the MARS (27%) which has more flexibility than the PARS (19%). The membrane roof flexibility also modifies the non-Gaussian characteristic which results in a lower peak factor than the plywood roof. It was also observed that peak pressure coefficients on the flexible roof increase at higher wind speeds. This study provides an improved understanding of the effect of roof flexibility on wind-induced pressure coefficients. Further research is needed by testing more types and configurations of flexible roofs to formulate new code provisions for wind effects on flexible membrane roofs.

Wind loads on roof-mounted equipment on low-rise buildings with low-slope roofs

Damage surveys have indicated that roof-mounted equipment has significant vulnerability to extreme windstorms. The study investigates anchorage and envelope wind loads on 1:50 scale, roof-mounted equipment. Cubes with equivalent full-scale side lengths of 0.9 ​m, 3 ​m, and 4.6 ​m, and a rectangular prism with dimensions of 6 ​× ​3 ​× ​1.5 ​m (length ​× ​width ​× ​height) are examined. Experiments were conducted with this equipment placed at four different locations, which represent the roof corner, perimeter, and field regions. Additionally, the equipment was also tested when flush-mounted and elevated to 1.5 ​m, 3 ​m, and 4.6 ​m above the roof surface. It is found that wind loads relevant for anchorage design are a function of the size of the equipment, elevation above the roof, and roof location. The uplift wind loads for anchorage design of flush-mounted and elevated equipment are different, irrespective of the equipment size because of the exposed bottom surface for elevated equipment. However, the elevation effect on drag wind loads for anchorage design is negligible. The envelope wind loads are distinct from both the anchorage loads and the cladding loads on bare-roofed, low-rise buildings due to differences in the associated local aerodynamics.

An investigation of snow drifting on low-sloped gable roofs: Wind-tunnel tests and CFD simulations

In this paper, a series of wind-tunnel tests and steady Reynolds-averaged Navier–Stokes (RANS) simulations were conducted to investigate the influence of roof slope upon snow drifting and flow characteristics around low-sloped gable roofs. First, the fresh-dry-snow-drifting processes on four gable roofs with inclinations of 5°, 10°, 15° and 20° were investigated via wind-tunnel tests using silica sand, and three reference wind velocities (namely 4.7 m/s, 5.85 m/s and 7.0 m/s) were taken into consideration. The snow-redistribution, peak-position, trapping-efficiency and transport-rate characteristics on model roofs were carefully discussed. The results of the wind-tunnel tests showed that roof slope significantly influenced snow depth on the leeward surface, that trapping efficiency increased along with roof slope, and that snow depth on the windward side was more affected by wind velocity. Sensitivity analyses for grid resolutions and inflow profiles were performed; the distributions of friction velocity, streamlines, positions of time-averaged vortex cores, and turbulence kinetic energy around buildings were investigated via numerical simulations. The change in the wind-flow pattern between inclinations of 10° and 15° resulted in the greatest transport-rate difference of all adjacent roof slopes. Finally, an empirical equation was provided that allowed the differential value between friction velocity and threshold friction velocity to estimate the snow-transport rate on the building roof.

Airflow over low-sloped gable roof buildings: Wind tunnel experiment and CFD simulations

In this study, the impact of roof slope on the flow characteristics over low-sloped gable roofs was investigated using steady computational fluid dynamics (CFD) simulations based on a k-ω SST turbulence model. A measurement database of the flow field over a scaled model of 15° was created using particle image velocimetry (PIV). Sensitivity analyses for the grid resolutions and turbulence models were performed. Among the three common Reynolds-averaged Navier-Stokes equations (RANS) models, the k-ω SST model exhibited a better performance, followed by the RNG model and then the realizable k- model. Next, the flow properties over the differently sloped (0° to 25°) building models were determined. It was found that the effect of roof slope on the flow characteristics was identified by changing the position and size of the separation bubbles, 15° was found to be approximately the sensitive slope at which the distribution of the separation bubbles changed significantly. Additionally, it is suggested additional attention focused on the distributions of the negative pressure on the windward surfaces (especially 5° and 10° roofs) and the possible snow redistribution on the leeward surfaces.

Development of chi-factors towards codification of the thermal bridging in low slope roofing assemblies

An industry consortium study, Energy Resistance of Commercial Roofs (ERCR), has been developed by NRC to develop scientifically supported performance data on energy resistance of roofing systems that are constructed according to field practices. The ERCR consortium project had two major tasks. In Task 1, the objective was to evaluate the effective thermal resistance of current roof designs and validate their compliance with the energy code requirements and, in Task 2, the aim was to quantify the two thermal impact factors – thermal bridging from fasteners and thermal bypass from gaps between the insulation boards. This paper focuses on the Task 2-thermal bridging from fasteners. More than 100 experiments were conducted to quantify the impact of fastener density, fastener location, fastener diameter, and fastener penetration depth on the thermal performance of the roofing assembly. From the experimental data, the thermal bridging of fasteners was quantified in terms of the “Relative” decrease in effective R-value and Chi-factors (χ). The impact of fastener density revealed that the thermal bridging increases with the fastener density and also with the thermal resistance of the insulation with an average loss in effective R-value ranging from 5% to 14% for the tested configurations. The increase in the fastener diameter or the fastener area (#12 < #14 < #15) increases the overall heat flow through the assembly. In a multilayer insulation layout, fasteners installed in the bottom layer are shielded by the top insulation and can reduce thermal bridging from 30% to 70% relative to the through fastener from the top layer. From the experimental data, chi factors were developed to support calculation of point thermal bridging effects on the thermal performance of the commercial roofs, and also to fill in the missing gaps in the energy codes towards the thermal design of roofs.

Wind loads on low-slope roofs of buildings with large plan dimensions

Wind load provisions in the North American codes and standards incorporate the results of wind tunnel studies and the full-scale measurements of wind loads. These studies examined wind effects on buildings with regular plan size, i.e. less than 60 m. This fact emphasizes the importance of testing large buildings in the wind tunnel and highlights the necessity of investigating the applicability of the North American code and standard provisions to large buildings. In the present study, three building models were constructed at a length scale of 1:400 with identical plan dimensions (equivalent to 118 m × 118 m) and different heights (equivalent to 5 m, 10 m and 20 m). The models were tested in simulated open country and suburban exposures. The experimental results were compared with full-scale data and the wind provisions of the North American codes and standards. It was found that the external peak pressure coefficients recommended by ASCE 7-16 (2017) are much higher than the experimental findings whereas those recommended by ASCE 7-10 (2010) are consistent with the experimental results. However, the experimental results indicate that the corner zone should be better defined as an L-shape for large buildings of 8 m height or more and the wind loads developed on the roof corner are approximately equal to those on the edge zone for large buildings of height less than 8 m.

Smart energy harvesting performance of photovoltaic roof assemblies in Canadian climate

ABSTRACT In Canada, the solar electricity sector is growing rapidly. Much of this success is based on the growth of the Ontario solar market where more than 99% of Canada’s solar electricity is generated. Ontario has developed a globally recognized solar market sector. The vast surface area of existing residential roofs across Canada represents an untapped resource for capitalizing on passive and active management of impinging solar insolation. The aim of the current research study is to evaluate the new energy harvesting technologies such as a thin-film PV integrated roof system that could serve as a conventional roofing for weather protection while generating clean solar electricity, and the new generation micro inverters that have the potential to outperform string inverters under shading and snow-cover conditions. This paper has two parts that will discuss about two smart energy harvesting technologies and their performance on residential applications in Canadian climate. Part 1 of the paper focusses on field evaluation of Roof Integrated Photovoltaic (RIPV) and Part 2 talks about the energy yield performance of integrated solar tiles and new generation micro inverters. The RIPV field trial took place at the Canadian Centre for Housing Technology (CCHT) Info Centre in Ottawa, Canada. This is a novel approach adapted from a roofing system that would typically be found on low-sloped roofs such as commercial supermarkets, industrial warehouses and school buildings. Over the eight month study period, surmounting the effects of snow cover and shadows, the RIPV system generated over 1 MWh of electricity, and had a measured system efficiency of 5.3%. The study on the new generation micro inverters for residential applications addressed the shading effects on the intermittent nature of solar energy generation. Simulating the shading conditions that are experienced by typical residential rooftop, the micro inverters were found to increase production by 1–68% relative to the conventional string inverters. The research outcome of this study has demonstrated that both these energy harvesting technologies have important incremental benefits in increasing the renewables contribution to power generation in residential homes in Canadian climatic conditions.

Eliminating Fastener Thermal Bridging in Low Slope Roofs: Energy Efficiency Savings versus Installation Costs

Low-slope roofing assemblies include a wide range of insulation and single ply membrane attachment methods. Previous studies have shown that mechanical attachment using metal fasteners leads to significant thermal bridging and consequent loss of insulation value and reduction of long term thermal efficiency. This study calculates the costs associated with mechanical attachment in terms of lost insulation value, increased long-term energy costs, and the material and labor costs associated with installation of these common systems. Energy efficiency reductions using metal fasteners were modeled for eight US cities in a range of climate zones. From the data, it was possible to calculate target costs (labor plus materials) that would make adhered systems cost effective. There are many options available to adhere single ply roof system components, such as solvent-based adhesives and low-rise urethane foams, together with different application approaches such as broom, spray, and ribbon methods. The cost targets derived in this study can help optimize the use of such materials and application methods such that the thermal bridging due to fasteners could be substantially reduced or eliminated.

Component and Cladding Wind Loads for Low-Slope Roofs on Low-Rise Buildings

AbstractThe component and cladding wind load provisions for low-sloped roofs on low-rise buildings in ASCE 7-10 were examined using measured pressure data from an aerodynamic database. It was found...

Study of a super-non-wetting self-cleaning solar reflective blue-grey paint coating with luminescence

A method for generating a solar reflective blue-grey paint coating with yellow-green luminescence that is cost-effective, industrially scalable, widely applicable, easily repairable, super-non-wetting, self-cleaning, and solar reflective, is introduced. The coating can be manufactured using as-received commercially available materials and following a simple industrial procedure. Grinding the coating surfaces using emery papers with appropriate grit numbers can readily yield appropriate surface roughness, generate suitable micro-grooves and expose micro- and nano-particles on the coating surfaces, thus endowing the coating surfaces with super-non-wettability and self-cleaning property. The surface free energy of the coating only depends on the dispersive component and both the Lewis acid component and the Lewis base component are 0mN/m. The solar reflective blue-grey coating with solar reflectance of 0.808 can yield a cooling effect of 22.6°C relative to a flat or low-sloped concrete roof on a typical clear summer sunny day. It emits yellow-green luminescence during the night, thus decorating the buildings to some extent when applied to the external walls of buildings. Although the super-non-wettability of the coating surfaces might be destroyed mainly by the periodic spraying of water (simulated by 400h of artificial accelerated weathering tests), the super-non-wettability and the self-cleaning property can be completely re-established after roughening the weathered coating surfaces using emery papers with appropriate grit numbers. In terms of optical properties, the solar reflective blue-grey coating with yellow-green luminescence has good resistance to artificial accelerated weathering.

Ventilation of Low Slope Roof Systems in Northern Climates

For over generations, multi residential (flats) buildings were constructed in North American with low slope roofs. The roof design included an interstial space of 1.5 to 2 meters between the ceiling and roof deck. This ‘attic’ space was typically vented via static air vents. The ceilings were typically constructed of plaster with little or no insulation. The roofing was typically asphalt or coal tar built up roofing applied directly to the roof deck. This simple ceiling to roof arrangement worked for centuries. In circa 1998, with the advent of mandated reflective roofs in several large North American cities, a simply workable design was changed, often with disastrous results. So that greater floor to ceiling heights could be attained, the 1.5 to 2 meter attic was reduced to .33 m or smaller. Plaster ceilings were changed to gypsum board. Fiberglass insulation with an ineffectual vapor barrier was installed at the ceiling, and often punctured by ‘can’ lights which allowed air transport of warm air, often containing moisture, into the attic cavity. With no attic ventilation, insulation on the roof deck, condensation accumulation on the attic structural elements to such a degree that, in many situations, resulted in the deterioration of the wood structural elements.This paper will review the importance of proper design with ‘modern day’ low slope roof systems with ceilings on the roof structure, and the role that venting, vapor retarders and roof color have on the success or failure of these roof systems. The author will review historical examples of recent failures and provide designers with recommendations for appropriate low slope residential roof system design.